RADIUM AND RADON MOBILITY IN FRESH GROUNDWATER: INSIGHTS ALONG SOLUTE AND REDOX GRADIENTS IN THE NORTH CAROLINA PIEDMONT

Radium isotopes, radon, major ions, and trace metal concentrations were investigated in groundwater from 97 private wells in fractured crystalline rocks (granite, granitic gneiss, metasedimentary rocks, and metavolcanic rocks) in Wake County, North Carolina. Most of the waters contain bicarbonate as the dominant anion (range 9-229 mg/L). The geochemistry shows a continuum between two major end members: 1) low-pH (5.0-6.0), low-TDS, low-HCO₃^- waters with dissolved oxygen (DO) near saturation; and 2) higher-pH (6.5-8.6), higher-TDS, elevated-HCO₃^- waters with low DO and typically higher Fe and Mn. The major element chemistry is probably acquired during recharge through the soil and saprolite, possibly influenced by organic carbon respiration, as increasing HCO₃^- is associated with decreasing DO and δ¹³C. Elevated radionuclides were encountered in many wells, especially in the Rolesville granite, with 72% of samples in excess of the proposed EPA radon standard of 4,000 pCi/L (148,000 mBq/L). In the same geological unit, only 3% of samples exceeded the EPA standard for ²²⁶Ra + ²²⁸Ra of 5 pCi/L (185 mBq/L). In general, high Rn is seen in the high-DO, low-HCO₃^- waters, and elevated Ra is seen in higher-HCO₃^-, lower-DO waters. The ²²²Rn/²²⁶Ra activity ratio increases with increasing DO, ranging from about 200 to 60,000. In addition, ²²²Rn/²²⁶Ra is inversely related to dissolved Mn and Fe. The ratios ²²³Ra/²²⁶Ra and ²²⁴Ra/²²⁸Ra are relatively close to the expected equilibrium values of 0.046 and 1, respectively. These ratios argue against a strong recoil contribution, rapid adsorption, and/or dissolution of primary minerals to explain Ra mobilization, as those processes could cause fractionation of short-lived relative to long-lived Ra isotopes. Our data suggest that both rock type (and thus surface content of parent radionuclides) and groundwater geochemistry influence Ra occurrence in Piedmont groundwater.